INVESTIGATED: Expert Mohd Khairul Amri Selamat is thought to have known how to disable the plane's tracking system [EPA]

Investigators believe passenger Mohd Khairul Amri Selamat knew how to disable the tracking systems and are probing whether he was a hijacker.

The flight engineer, who worked for a jet charter firm, was one of 239 passengers and crew aboard when the Malaysian Airlines jet disappeared 10 days ago.

A senior police official said: “We are looking into Mohd Khairul as well as the other passengers and crew. The focus is on anyone who might have had aviation skills.”

Dad-of-one Khairul, 29, worked for a Swiss-based jet charter firm called Aviation Group, but the company declined to say if it still employed him. In a picture posted on Khairul’s Facebook page in 2011, he said he was an employee of Execujet in Malaysia.

A spokesman for the company said: “We can’t disclose anything. We want to protect the family’s privacy.”

SIGN OF HOPE: Philippines school art to bring luck for the search [REUTERS]

His dad Selamat Omar, 60, last night insisted that his son had nothing to do with the plane’s disappearance.

He said: “My son would have done no wrong.”

One line of inquiry pursued by investigators is whether the jet flew to Taliban territory on the Afghan and Pakistani border.

Pakistani officials said radar recordings had no sign of the jet.

The investigation has up to now been focused on pilot Captain Zaharie Ahmad Shah and first officer Fariq Abdul Hamid.

Fears of a hijack deepened when it emerged Hamid said: “All right, good night,” after someone had disabled the tracking systems.

The plane vanished from radar shortly after take-off from Kuala Lumpur at 12.40am on March 8, on a flight to Beijing.

Signals from the jet were last received at 8.11am. Officials say it could have been on the ground when it transmitted them.